Before DNA and proteins, life may have run entirely on RNA
Every living cell needs DNA to store instructions, RNA to carry them and proteins to do the work, and none functions without the others. That chicken-and-egg puzzle led scientists to propose an earlier era when one molecule did both jobs. The strongest clue sits inside every cell today: the ribosome.
The idea is called the RNA world. Alexander Rich sketched it in 1962, Francis Crick, Leslie Orgel and Carl Woese floated similar thoughts, and Walter Gilbert gave it its name in 1986. RNA can carry genetic information much as DNA does, and in the 1980s researchers discovered that it can also act as a catalyst. These RNA enzymes, called ribozymes, still operate in modern organisms and may be living fossils of that ancient stage.
The ribosome, which builds every protein, is made mostly of RNA, and its bond-forming core appears to be pure RNA chemistry: no amino acids sit within 18Å of the active site, and a ribosome stripped of most of its protein still joins amino acids. A short lab-made RNA did the same trick in 1999, and in 2022 a molecule modelled on part of ribosomal RNA paired up and formed peptide bonds, supporting the notion of an ancient proto-ribosome. Several key coenzymes also look strikingly like fragments of RNA.
The hard part is showing RNA copying itself. Lab-made ribozymes as short as 165 bases can assemble other RNA strands, and one 189-base version made just 1.1% errors per nucleotide, though it could only copy templates too short to reproduce itself. In 2016, rounds of test-tube evolution yielded a polymerase called 24-3 able to copy almost any other RNA.
If an RNA world existed, it was probably succeeded by a mixed era of RNA and protein, then by DNA, which is sturdier for storage, and proteins, whose many building blocks make more versatile enzymes. Some researchers suspect an even earlier pre-RNA came first. Life itself arose more than 3.5 billion years ago, and though no conclusive proof rules out rival ideas, the RNA world remains the most favoured account of how it began.
Source: RNA world
